This paper investigates the particularities of vocal communication betweenmothers who suffer from Borderline Personality Disorder (BPD) and theiryoung infants. One of the characteristic signs of BPD is a marked difficulty insustaining long-term interpersonal relationships. Social interactions betweenmothers with BPD and their 3-month-old infants were found to include lessinfant participation, longer interruptions, more repetition and unvoiced sounds.The study presented in this paper highlights prosodic specificities in BPD mothers&#8217;speech to infants as well as a lack of sensitivity to infants&#8217; emotional statesreflected in the content of mothers&#8217; speech. It sheds light on the role of sharedemotion in enabling the first forms of human social interaction, before infantscan speak.